Prior art solenoid actuators used to control the flow of fluid through a valve are typically of two distinct types. The most common type is a two-position actuator that includes a plunger element that moves between two positions. The plunger is in a first position when no current flows through the coil of the actuator. When an electric current flows through the coil, the plunger element is moved by electromagnetic force to a second position. When the current flow is stopped, the plunger is returned by a spring to its original position. Such "on-off" actuators are commonly used on two position fluid flow control valves.
A second type of solenoid actuator is used in proportional valves. Such actuators include a plunger or other type of movable element that may be selectively moved through a range of positions in response to control signals delivered to the coil of the actuator. Proportional actuators are used in valves that meter the rate of flow.
In both types of prior art solenoid actuators, certain problems exist. The plunger elements used in the actuators are constrained to move in a tubular shaped housing. The magnetic forces that move the plunger element in the desired axial direction, also produce lateral forces. These forces tend to push the plunger against the walls of the housing which increases friction. This frictional resistance increases the amount of force that is required to move the plunger element and correspondingly requires the coil to produce more power to move the plunger.
Another problem with the prior art actuators is that the frictional forces on the plunger element impair accurate positioning of the plunger element, and the flow control elements in the valve to which the plunger element is connected. This is particularly a problem with proportional actuators which exhibit hysteresis. The frictional forces limit the ability of the plunger element to move to the same location in response to identical control signals.
Prior art solenoids have used a technique called "dither" to slightly oscillate the plunger element and reduce friction. However, the problems associated with friction still persist. Others have used ball bearing supports for the plunger element in an attempt to reduce friction, such as the supports shown in U.S. Pat. No. 4,525,695. While this approach has been helpful, the structure of the supports still creates an undesirable level of friction. This is particularly true because the movable element is subjected to sliding friction at one point on each ball bearing despite the existence of the ball bearing type supports.
Thus there exists a need for a solenoid actuator that provides reduced friction, requires less power and achieves more accurate control.